For several decades nitroglycerin has been administered to humans as a vasodilating agent in the treatment of cardiovascular disease. It has been shown that nitroglycerin so administered is converted in the body to nitric oxide which is the pharmacologically active metabolite. Recently, nitric oxide has been shown to be formed enzymatically as a normal metabolite from arginine in vascular endothelium to provide an important component of endothelium-derived relaxing factors (EDRFs) which are currently being intensively studied as participating in regulation of blood flow and vascular resistance. Macrophages have also been shown to produce nitric oxide in the body as a component of their cell killing and/or cytostatic function.
More recently it has been established that the enzyme forming nitric oxide from arginine, i.e., nitric oxide synthase, occurs in at least two distinct forms, namely a constitutive form and an inducible form. The constitutive form is present in normal endothelial cells, neurons and some other tissues. Formation of nitric oxide by the constitutive form in endothelial cells is thought to play a role in normal blood pressure regulation. The inducible form of nitric oxide synthase has been found to be present in activated macrophages and is induced in endothelial cells and vascular smooth muscle cells, for example, by various cytokines and/or microbial products. It is thought that in sepsis or cytokine-induced shock, overproduction of nitric oxide by the inducible form of nitric oxide synthase plays an important role in the observed life-threatening hypotension. Furthermore, it is thought that over-production of nitric oxide by the inducible form of nitric oxide synthase is a basis for insensitivity to clinically used pressor agents such as .alpha..sub.1 adrenergic agonists in the treatment of septic or cytokine-induced shock patients.
Considerable research effort has been expended to discover inhibitors for nitric oxide synthase activity. Much of this research effort has been directed at uncovering arginine antagonists to function to inhibit nitric oxide synthase activity. Most of the inhibitors uncovered thus far block not only inducible nitric oxide synthase activity but also constitutive nitric oxide synthase activity; and typically any specificity of inhibition of any particular arginine antagonist for inducible nitric oxide synthase activity is not so high that it is possible to block hypotension-causing, pathological overproduction of nitric oxide (an inducible enzyme-mediated process) to a therapeutically adequate extent (i.e. so that clinically serious hypotension that would normally occur in sepsis or cytokine-induced shock is avoided or so that pressor agent sensitivity is restored), and, at the same time, not block the physiological nitric oxide synthesis which is thought to play a role in neural function and normal blood pressure regulation (constitutive enzyme-mediated processes) and thereby avoid the toxicity (e.g., neuronal toxicity and hypertension) associated with interfering with physiological nitric oxide synthesis.